1. Field of the Invention
The invention relates in general to a calibration method, and more particularly to a calibration method for improving stability of a write control signal during writing.
2. Description of the Related Art
FIG. 1 is a block diagram showing a conventional CD recorder. Referring to FIG. 1, the CD recorder mainly has a read control device 10, a write control device 20 and a pickup head device 30. When a data reading process is to be performed, the read control device 10 generates a read control signal VRDC and outputs the signal VRDC to a drive IC 304 of the pickup head device 30. The drive IC 304 generates a current iD flowing through a laser diode 301 according to the read control signal VRDC so as to generate a laser beam to read a disk 40. When a data writing process is to be performed, the write control device 20 generates a write control signal VWDC and outputs the signal VWDC to the drive IC 304 of the pickup head device 30. The drive IC 304 generates a current iD according to the write control signal VWDC so as to generate a laser beam to write the disk 40.
Besides, A monitor diode 302 of the pickup head device 30 generates a current iM according to the laser beam sensed by the laser diode 301. A third amplifier 303 processes the current iM and then generates a feedback control signal FPDO. The feedback control signal FPDO is fed back to input terminals of the read control device 10 and the write control device 20 in order to control a light-emitting power generated by the laser diode 301 during the reading/writing process.
Please refer to the Taiwan patent application number 92100819, filed on Jan. 15, 2003 and entitled “Write Control Device and Method for Optical Storage Apparatus”, which illustrates write control device 20 which has three working modes including a short-term open mode, a long-term open mode and a close-loop mode, as respectively shown in FIGS. 2, 3 and FIG. 4. In the short-term open mode shown in FIG. 2, the write control signal VWDC is initialized according to a virtual ground of a first amplifier 201, so that the potential of the write control signal VWDC become to zero. In the long-term open mode shown in FIG. 3, an analog signal DAC2 with a voltage level of V1 is inputted to a second amplifier 203. The gain G of the second amplifier 203 is V2/V1, and the second amplifier 203 outputs an output signal with a voltage level of V2 to the first amplifier 201 such that the first amplifier 201 outputs the write control signal VWDC of which the voltage level becomes V2. The analog signal DAC2 is the signal which influences the level of the write control signal VWDC. The analog signal DAC2 may be obtained using a DAC converter to convert a digital value. The digital value may be selected from one of a plurality of digital values stored in a memory. Different values determine different levels of the write control signal VWDC. In this long-term open mode, the write control signal VWDC can be rapidly charged to a voltage level V2 capable of performing writing toward the optical disk 40. In the close-loop mode in FIG. 4, after the feedback control signal FPDO is inputted to the write control device 20, a sample/hold unit S/H samples and holds the signal FPDO and amplifies the signal FPDO by G12 times. Then, the amplified signal is inputted to a negative input terminal of the first amplifier 201 through a resistor Ri2, while the analog signal DAC2 with a voltage level of V1 is directly fed to a positive input terminal of the first amplifier 201. The first amplifier 201 outputs the write control signal VWDC with a voltage level of V2′ and feeds the signal VWDC back to the negative input terminal of the first amplifier 201 across the parallel connected resistor Rf2 and capacitor C2. In this close-loop mode, the write control signal VWDC controls the pickup head device 30 to write data into the optical disk 40.
Obviously, In an ideal state, V2′ described above should be equal to V2. FIG. 5A shows signal waveforms of a time interval control signal WLDON and the write control signal VWDC when V2′ is equal to V2. The time interval control signal WLDON is used to control the write control device 20. When the time interval control signal WLDON switches to a high level, the write control device 20 switches to the close-loop mode to perform the writing operation. Therefore, in the time slot T1, the time interval control signal WLDON is low, the write control device 20 enters the long-term open mode, and the level of the write control signal VWDC is V2. In the time slot T2, the time interval control signal WLDON switches to the high level, the write control device 20 enters the close-loop mode, and the level of the write control signal VWDC is V2′. Consequently, the write control signal VWDC is quickly charged to the voltage level V2 capable of writing the optical disk 40 in the long-term open mode, and then the write control signal VWDC is held at the voltage level V2′ capable of writing the optical disk 40 (in an ideal state, V2′=V2) in the close-loop mode. Thus, the write error may be avoided when the optical drive is being written.
However, if the gain of the second amplifier 203 is not well designed, or the gain G of the second amplifier 203 is changed due to the influence of the environmental factor or temperature, V2′ will not be equal to V2. FIG. 5B shows signal waveforms of the time interval control signal WLDON and the write control signal VWDC when V2′ is not equal to V2. When V2′ is not equal to V2 and the write control device 20 switches from the long-term open mode in the time slot T1 to the close-loop mode in the time slot T2, the write control signal VWDC may have a transient of unstable voltage, which may cause write error when the optical drive is writing the disk.
Besides, the conventional method for obtaining the gain G of the second amplifier 203 will be described in the following. An OPC (Optimum Power Control) calibration is firstly performed on a test area of the disk before the disk is written. The OPC calibration is to write the test area according different light-emitting powers of the laser diode. In the OPC calibration process, a most suitable light-emitting power for writing may be obtained, and the theoretical value of the gain of the second amplifier 203 may be evaluated according to the light-emitting power. Thereafter, the fixed theoretical value of the gain serves as the gain G of the second amplifier 203. In a CAV (Constant Angle Velocity) optical drive, however, when the writing speeds are different, different light-emitting powers have to be set. That is, different gains G have to be set to obtain different voltage levels V2 of the write control signal VWDC. Obviously, the conventional method cannot achieve the function of changing the gain G according to the writing speed, so this method may cause the unstable voltage of the write control signal VWDC and thus cause the data write error.